1. Field of the Invention
The invention relates to a mechanism for releasing a wireline cable from a cable head.
2. Background Art
In oil and gas wireline operations, downhole tools, e.g., logging tools, are conveyed downhole within a wellbore using a wireline cable. The downhole tools are typically tubular members that are threaded together to form a xe2x80x9ctool string.xe2x80x9d A cable head couples the wireline cable to the tool string. Occasionally, during operation, the tool string may become stuck in the wellbore. When the tool string gets stuck, a high tension is usually applied to the tool string to try to free the tool string from its stuck position. This high tension is applied to the wireline cable at the surface, and the wireline cable transmits the applied tension to the cable head. The cable head in turn transmits the tension to the tool string. The amount of tension available to free the tool string from its stuck position depends on the breaking strength of the wireline cable, the profile and coefficient of friction of the wellbore, the position of the tool string inside the wellbore, and various other parameters, in particular the weight of the cable in the wellbore.
The connection between the cable head and the wireline cable typically includes a xe2x80x9cweak point.xe2x80x9d A weak point is a link designed to break when a predetermined amount of tension is applied to it. Normally, the weak point has the lowest breaking strength in the tensile string. The weak point allows the wireline cable to be separated from the cable head in the event that enough tension cannot be applied to free the tool string. The operator first latches onto the cable head or tool string using a fishing tool coupled to one end of a drill pipe and then applies tension to the wireline cable to break the weak point and release the wireline cable from the cable head. The wireline cable is first removed from the wellbore, and hen the cable head and the tool string are pulled out of the wellbore by removing the drill pipe.
The weak point is usually designed for the worst case scenario. In other words, the breaking strength of the weak point must be lower than the minimum tension that the wireline cable can transmit to the desired maximum depth of descent into the wellbore. Otherwise, if the tool string gets stuck at a depth where the amount of tension that can be transmitted safely through the wireline cable is less than the breaking strength of the weak point, it will be impossible to break the weak point. The strength of the weak point must also be greater than the weight of the tool string plus a safety factor. These requirements sometimes limit the depth to which the tool string can safely descend inside the wellbore.
However, the tool string will not always get stuck at the maximum depth of descent into the wellbore. If the tool string gets stuck at a point above the maximum depth of descent into the wellbore, the maximum tension that can be transmitted to the cable head without breaking the wireline cable will be much greater than what is needed to break the weak point. If this maximum tension is transmitted to the cable head, the weak point will break before the fully available tension can be used to try to free the tool string.
The different tool sticking conditions give rise to the need for a weak point having two distinct breaking strengths. In one mode, the breaking strength of the weak point is greater than the breaking strength of the wireline cable so that all the tension capable of being transmitted to the cable head can be applied to freeing the tool string from its stuck position. In another mode, the weak point can be broken without exceeding the breaking strength of the wireline cable at any depth of descent.
U.S. Pat. No. 6,032,733 issued to Ludwig et al. discloses a latch assembly for releasably connecting a wireline cable to a cable head which operates in two modes. The latch assembly includes an anchor sub keyed within an inner housing. The anchor sub has a tensile strength greater than the safe pull of the wireline, where xe2x80x9csafe pullxe2x80x9d of the wireline is defined as a tension that does not exceed one-half the breaking strength of the wireline. The anchor sub has a neck portion and a bore extending from the neck portion to the body of the anchor sub. A latch housing is threaded to the neck portion of the anchor sub, and a chamber is defined within the latch housing. A latch shaft extends through the chamber. One end of the latch shaft is coupled to the wireline. The latch shaft has an enlarged portion which divides the chamber into two sub chambers. In the latched position, there is an interference fit between the latch shaft and the latch housing, the upper sub chamber contains a fusible material, and the volume of the lower sub chamber is substantially zero. The latch assembly also includes heaters for heating the fusible material.
During normal operation, the latch assembly couples the wireline to the cable head housing. When it is desired to release the wireline from the cable head, the operator sends a command to a switching circuit which then directs current to the heaters. The heaters, which are in contact with the latch housing, heat the metal of the latch housing, causing the latch housing and the enlarged portion of the latch shaft to expand. The latch housing has a higher coefficient of expansion than the enlarged portion of the latch shaft. Thus, a gap is formed between the latch housing and the enlarged portion of the latch shaft as the latch housing is heated. The heated latch housing also causes the fusible material in the upper chamber to melt. The melted fusible material flows into the lower sub chamber through the gap formed between the latch housing and the enlarged portion of the latch shaft. As the fusible material flows into the lower sub chamber, the tension applied to the latch shaft by the wireline cable causes the latch shaft to move upwardly. This causes the latch shaft to move to the unlatched position.
In one aspect, the invention relates to a cable release apparatus which comprises a housing and a latch mounted at one end of the housing. The latch has a central opening and a plurality of projecting members extending into the housing. A releasable connector is mounted inside the housing. An actuator has one end disposed in the central opening in the latch and another end in contact with the releasable connector. The actuator is movable between a first position prior to activation of the releasable connector and a second position wherein the releasable connector is activated. Prior to activation of the releasable connector, the latch is held in place by an interference fit between the projecting members and the housing. When the releasable connector is activated, the projecting members are deflected by applying tension to the latch, thereby releasing the latch from the housing.
In some embodiments, the projecting members comprise outer wedged surfaces for engagement with an inner wedged surface on the housing. In some embodiments, the projecting members comprise inner wedged surfaces for engagement with an outer wedged surface on the actuator. In some embodiments, the releasable connector comprises a plurality of connector segments held together by a spring and a heater for heating a solder joint in the spring so as to enable expansion of the spring. In some embodiments, a spring is provided to apply a force on the actuator such that the actuator moves in the direction of the releasable connector when the releasable connector is activated.
In another aspect, the invention relates to a cable head which comprises a head housing and a cable release housing mounted inside the head housing. The cable head further comprises a latch mounted at one end of the cable release housing. The latch has a central opening and a plurality of projecting members extending into the cable release housing. The cable head further comprises a cable connector coupled to the latch. A releasable connector is mounted inside the cable release housing and an actuator has one end disposed in the central opening in the latch and another end in contact with the releasable connector. The actuator is movable between a first position prior to activation of the releasable connector and a second position wherein the releasable connector is activated. Prior to activation of the releasable connector, the latch is held in place by an interference fit between the projecting members and the housing. When the releasable connector is activated, the projecting members are deflected by applying tension to the latch, thereby releasing the latch from the housing.
In another aspect, the invention relates to a logging tool which comprises a head housing and a cable release housing disposed inside the head housing. The cable head further comprises a latch mounted at one end of the cable release housing. The latch has a central opening and a plurality of projecting members extending into the cable release housing. The cable head further comprises a cable connector coupled to the latch. A releasable connector is mounted inside the cable release housing and an actuator has one end disposed in the central opening in the latch and another end in contact with the releasable connector. The actuator is movable between a first position prior to activation of the releasable connector and a second position wherein the releasable connector is activated. A downhole tool is coupled to the head housing and the cable release housing. Prior to activation of the releasable connector, the latch is held in place by an interference fit between the projecting members and the cable release housing. When the releasable connector is activated, the projecting members are deflected by applying tension to the latch.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.